Electricity distribution networks (or grids) are often supported by a fleet of diesel generators and open cycle gas turbines that provide electricity during periods of high demand and emergency events such as the unexpected failure of a power station. Such generating assets, often referred to as peaking plant, burn fossil fuels at low efficiency and can be a significant source of atmospheric pollutants. The services provided by such peaking plant, include, but are not limited to,                balancing differences in supply and demand at different times of the day and at short notice,        providing electricity required to power the auxiliary equipment required for restart of a generating asset in the event of total network failure (black-start support),        network reinforcement where parts of the electricity distribution network have a shortfall in capacity during periods of high power demand,        injecting power into the network to support the frequency of the grid when demand for electricity increases rapidly.        
In addition, the loss of power from the electricity distribution network can result in significant economic loss to some consumers, such as a data centre, or danger to personnel, for example in the event of a power failure at a hospital. Such applications often utilise diesel generators to provide standby electricity in the event of an interruption to the supply of electricity from the distribution network. Replacement of such diesel powered generators with a zero emissions device that uses a fuel from a sustainable source would be of benefit.
There is a need for a device that can provide a similar service but that uses a fuel that produces low or preferably zero atmospheric pollution that originates from a sustainable source.
The present inventors have realised that there is potential to generate electricity using the expansion of liquid air, liquid nitrogen or cryogen to drive a turbine to generate electricity. Such a device could provide a compact, reactive and environmentally clean solution to the problems of balancing network supply with demand.
WO 2007/096656 discloses a cryogenic energy storage system which exploits the temperature and phase differential between low temperature liquid air, liquid nitrogen or cryogen, and ambient air, or waste heat, to store energy at periods of low demand and/or excess production, allowing this stored energy to be released later to generate electricity during periods of high demand and/or constrained output. The system comprises a means for liquefying air during periods of low electricity demand, a means for storing the liquid air produced and an expansion turbine for expanding the liquid air. The expansion turbine is connected to a generator to generate electricity when required to meet shortfalls between supply and demand. The target applications for the present invention require a very low number of operating hours per year, typically less than 500 and in the case of back-up power applications, much less. It would be uneconomic to install a complete energy storage system to service such applications due to the low utilisation of the air liquefier equipment and relatively high cost of this equipment for such a low level of utilisation.
PCT/BR2006/000177 discloses a device for generating power from liquid air which utilises ambient heat to provide thermal energy for the evaporation process. The inventors believe that this solution is impractical as a very large area of heat transfer surface would be required to prevent the build-up of excessive ice on the evaporator during the evaporation of the cold cryogenic fluid.